Numerical Simulations of Astrophysical Jets from Keplerian Accretion Disks

Abstract

<p>This thesis presents a series of magnetohydrodynamic (MHD) simulations which<br />were designed to study the origin and evolution of astrophysical jets (galactic and<br />extra-galactic). \Ve developed and extended a version of the ZEUS-2D code which<br />served as the numerical basis of our simulations and attached to it a complete analysis<br />package that was developed in order to make contact with the theory and observations<br />of jets.</p> <p>With our version of the code, we managed to establish an initial state which<br />consists of an accretion disk and its cold corona in stable eqailibrium around a central object.<br />No softening paranleter was used to model the Newtonian gravitational<br />potential of the central object. The corona and accretion disk are initially in pressure<br />balance with one another. These initial states were constructed so as to be<br />numerically stable. The corona is Inagnetized with the magmetic field lines extending<br />smoothly into the disk without kinks or discontinuities, avoiding, in this way, any<br />undesired currents in the initial set up. The disk is set in Kepler rotation and gas is<br />continuously injected into the corona above at the very snlalll speed of 10⁻³ times the<br />Kepler velocity.</p> <p>In this thesis, we only considered magnetic configurations for which the Lorentz<br />force is initially zero (J x B = 0). In particular initial J = 0 configurations are<br />studied. \Ve carefully set the boundary conditions to be open conditions so as to avoid any collimation due to grid reflection effects.</p> <p>To test the theory of winds centrifugally driven from the surface of Keplerian accretion<br />disks, we started with an open magnetic field line configuration. The magnetic<br />field lines have opening angles (with respect to the disk surface) less than the critical<br />angle (≃ 60°), as required for a centrifugally driven wind to start. We found that a<br />steady jet is quickly established allowing direct comparison with the theory. We find<br />the gas to be centrifugally accelerated through the Alfvén and the fast magnetosonic<br />surfaces and collimated into cylinders parallel to the disk's axis. The collimation is<br />due to the pinch force exerted by the dominant toroidal magnetic field generated by<br />the outflow itself. The velocities achieved in our simulations are of the order of 250<br />km/s for our standard young stellar object (a 0.5 M proto-star) and of the order<br />or 10⁵ km/s for our standard active galactic nuclei (a 10⁸M black hole). Our jet<br />solutions are very efficient in magnetically extract.ing angular momentum and energy<br />fronl the disk.</p> <p>The second magnetic configuration we have studied consists of a uniform vertical<br />structure wherein the magnetic field lines are parallel to the disk's axis. Here, the<br />rotation of the disk twists the magnetic field lines and generates a toroidal field<br />component. Because of the Keplerian scaling of the rotational velocity with the disk<br />radius, the twisting of the field lines is higher in the inner parts of the disk. The<br />strong lnagnetic gradient thus generated opens up the initial magnetic configuration<br />in a narrow region located at 1rᵢ < r < 8rᵢ, with rᵢ being the innermost radius of the disk. Within this narrow region a wind is ejected from the field lines that have<br />opened to less than the critical angle (≃60°), as expected from the centrifugally<br />driven wind theory. Our simulations show that the strong toroidal magnetic field<br />generated recollimates the flow towards the disk's axis and, through MHD shocks,<br />produces knots. The knot generation mechanism occurs at a distance of about z ≃ 8rᵢ from the surface of the disk.</p> <p>We have discovered that no special initial magnetic field structure is required in<br />order to launch episodic outflows in our simulations. Rather, conditions favorable for<br />the formation of an outflow set themselves up automatically through the production<br />of a toroidal magnetic field whose pressure readjusts the structure of the field above<br />the disk. The knot generator is episodic, and is inherent to the jet. Thus, jets<br />are apparently capable of producing the variability that leads to episodic events, independently of the underlying source.</p>